Metallurgical coke is a carbon material resulting from the manufactured purification of multifarious blends of bituminous coal. In its natural form, bituminous coal is soft; its medium-grade composite contains a high occurrence of unstable components. The majority of the unstable components are either reclaimed or recycled.
The bituminous coal used in the production of metallurgical coke is found in small quantities in the United States but in larger quantities in Asian and Far Eastern countries, such as India and China. Large reserves have been found in Turkey, especially on the western coastal region of the Black Sea.
The purification process is performed in coke batteries, which are platforms of large enclosed ovens. Once bituminous coal is loaded into the ovens (in a vacuum), it is heated to approximately 1,000 degrees Celsius for more than 22 hours. During the heating process, the unstable components are released, and the remaining solid coal endures a partial melting and subsequent resolidification to a hard carbon. The unstable components include ammonia, coal tar and dozens of other waste products.
The resulting solid is a stable carbon known as metallurgical coke. Due to the elimination of unstable components and volatile gasses, and also due to the partial melting, "met coke," as it is sometimes called, has an open, porous structure and may appear glassy in some samples. Met coke has a very low waste product content; however, the "ash" elements that were part of the original bituminous coal remain trapped in the resultant coke. Met coke is available in a variety of sizes, from basketball-sized chunks to fine powder.
Metallurgical coke is used in applications requiring strong-wearing carbon of high performance, high quality and resilience. Some applications for met coke include foundry coatings, drilling applications, heat treatment, oxygen exclusion, electrolytic processes, conductive flooring, friction materials, foundry carbon raiser, corrosion materials, reducing agents, iron ore refining and ceramic packing media. Metallurgical coke is also an agent in the production of ferro-alloys, calcium carbide, elemental phosphorus and carbon electrodes.
The key characteristic of met coke is its stable burning temperature while producing little or no smoke. It is used as the critical reducing agent in smelting iron ore into pig iron, making it a key ingredient in the production of steel. More than 90 percent of the met coke produced is used in the iron and steel industries. |
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 Iron ores are rocks and minerals from which metallic iron can be economically extracted. The ores are usually rich in iron oxides and vary in color from dark grey, bright yellow, deep purple, to rusty red. The iron itself is usually found in the form of magnetite (Fe3O4), hematite (Fe2O3), goethite (FeO(OH)), limonite (FeO(OH).n(H2O)) or siderite (FeCO3). Hematite is also known as "natural ore", a name which refers to the early years of mining, when certain hematite ores containing up to 66% iron could be fed directly into iron-making blast furnaces. Iron ore is the raw material used to make pig iron, which is one of the main raw materials to make steel. 98% of the mined iron ore is used to make steel. Indeed, it has been argued that iron ore is "more integral to the global economy than any other commodity, except perhaps oil”.
Metallic iron is virtually unknown on the surface of the Earth except as iron-nickel alloys from meteorites and very rare forms of deep mantle xenoliths. Although iron is the fourth most abundant element in the Earth's crust, comprising about 5%, the vast majority is bound in silicate or more rarely carbonate minerals. The thermodynamic barriers to separating pure iron from these minerals are formidable and energy intensive, therefore all sources of iron used by human industry exploit comparatively rarer iron oxide minerals, the primary form which is used being hematite.
Prior to the industrial revolution, most iron was obtained from widely available goethite or bog ore, for example during the American Revolution and the Napoleonic wars. Prehistoric societies used laterite as a source of iron ore. Historically, much of the iron ore utilized by industrialized societies has been mined from predominantly hematite deposits with grades in excess of 60% Fe. These deposits are commonly referred to as "direct shipping ores" or "natural ores". Increasing iron ore demand, coupled with the depletion of high-grade hematite ores in the United States, after World War II led to development of lower-grade iron ore sources, principally the utilization of taconite in North America. Lower-grade sources of iron ore generally require beneficiation. Magnetite is often utilized because it is magnetic, and hence easily separated from the gangue minerals and capable of producing a high-grade concentrate with very low levels of impurities. Due to the high density of hematite relative to associated silicate gangue, hematite beneficiation usually involves a combination of crushing, milling, gravity or heavy media separation, and silica froth flotation. One method relies on passing the finely crushed ore over a bath of solution containing bentonite or other agent which increases the density of the solution. When the density of the solution is properly calibrated, the hematite will sink and the silicate mineral fragments will float and can be removed.
Iron ore mining methods vary by the type of ore being mined. There are four main types of iron ore deposits worked currently, depending on the mineralogy and geology of the ore deposits. These are magnetite, titanomagnetite, massive hematite and pisolitic ironstone deposits.
Banded iron formations (BIF) are metamorphosed sedimentary rocks composed predominantly of thinly bedded iron minerals and silica (as quartz). The iron mineral present may be the carbonate siderite, but those used as iron ores contain the oxides magnetite or hematite. Banded Iron formations are known as taconite within North America. Mining of BIF formations involves coarse crushing and screening, followed by rough crushing and fine grinding to comminute the ore to the point where the crystallised magnetite and quartz are fine enough that the quartz is left behind when the resultant powder is passed under a magnetic separator.
The mining involves moving tremendous amounts of ore and waste. The waste comes in two forms, bedrock in the mine (mullock) that isn't ore, and unwanted minerals which are an intrinsic part of the ore rock itself (gangue). The mullock is mined and piled in waste dumps, and the gangue is separated during the beneficiation process and is removed as tailings. Taconite tailings are mostly the mineral quartz, which is chemically inert. This material is stored in large, regulated water settling ponds.
The key economic parameters for magnetite ore being economic are the crystallinity of the magnetite, the grade of the iron within the BIF host rock, and the contaminant elements which exist within the magnetite concentrate. The size and strip ratio of most magnetite resources is irrelevant as BIF formations can be hundreds of metres thick, with hundreds of kilometers of strike, and can easily come to more than 3,000 million or more, tonnes of contained ore.
The typical grade of iron at which a magnetite-bearing banded iron formation becomes economic is roughly 25% Fe, which can generally yield a 33% to 40% recovery of magnetite by weight, to produce a concentrate grading in excess of 64% Fe by weight. The typical magnetite iron ore concentrate has less than 0.1% phosphorus, 3–7% silica and less than 3% aluminium.
The grain size of the magnetite and its degree of commingling with the silica groundmass determine the grind size to which the rock must be comminuted to enable efficient magnetic separation to provide a high purity magnetite concentrate. This determines the energy inputs required to run a milling operation. Generally most magnetite BIF deposits must be ground to between 32 and 45 micrometres in order to produce a low-silica magnetite concentrate. Magnetite concentrate grades are generally in excess of 63% Fe by weight and usually are low phosphorus, low aluminium, low titanium and low silica and demand a premium price.
Currently magnetite iron ore (taconite) is mined in Minnesota and Michigan in the U.S., and Eastern Canada. Magnetite bearing BIF is currently mined extensively in Brazil, which exports significant quantities to Asia, and there is a nascent and large magnetite iron ore industry in Australia. |
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